Orotidine-5'-phosphate Decarboxylase. We recently synthesized 1-(5'-phospho-beta-D-ribofuranosyl) barbituric acid, and discovered that it binds extraordinarily tightly to orotidine-5'-phosphate decarboxylase. The barbituric acid derivative appears to be an irreversible inhibitor, although it can be removed from denatured enzyme. The affinity column we developed (and discussed in the Progress Report) allows us to prepare the yeast enzyme in a state of purity for the first time. We propose to investigate the following aspects of the binding of the inhibitor to pure enzyme: (a) we will try to modify it so as to achieve affinity labeling of the active site: (b) we shall try to modify it to form a photoaffinity labeling reagent for the active site; (c) we shall try to measure the binding constant, and to determine whether this inhibitor is a transition state analog, with consequent mechanistic implications, and (d) we shall of course measure the conventional properties (pH-rate profile, kinetics) of the pure enzyme. Photoaffinity Labeling. An attempt will be made to photolabel the dopamine receptor site of calf brain caudate nuclei tissue with our newly synthesized azidodopamine. Attempts to label the dopamine receptor site with diazomalonylfluphenazine have so far proved unsuccessful; the experiments will be repeated and analyzed in an attempt to discover the reason for the difficulties, so as to carry the experiments to a successful conclusion.